The present invention relates to an instrument for conducting secondary-ion mass spectrometry (SIMS) and, more particularly, to a direct imaging type SIMS instrument.
Secondary-ion mass spectrometry involves bombarding a sample with a primary particle beam and analyzing the secondary ions that emanate from the sample surface. The secondary ions are then introduced into a mass analyzer, where they are mass analyzed. As a result, the composition of a microscopic region on the surface of the solid sample can be elucidated. Instruments for conducting SIMS are broadly classified into two types: scanning type which scans an analyzed region with a sharply focused primary beam to obtain an ion image; and direct imaging type which bombards the whole analyzed region with a primary beam of a relatively large diameter and obtaining an ion image on the principle of an ion microscope. In principle, the direct imaging type has a higher sensitivity than the scanning type, because the direct imaging type simultaneously detects the secondary ions emanating from the whole analyzed region.
FIG. 1 shows the ion optics of one example of the direct imaging type SIMS instrument. A primary ion beam I.sub.1 produced from an ion source IS has a relatively large diameter. This beam is caused to impinge on the whole analyzed region on a sample S. Secondary ions I.sub.2 emanating from this region are sent to a mass analyzer MS through a transfer optics TO. In this mass analyzer, only secondary ions having a certain mass are selected and then projected via a projector lens Lp onto a two-dimensional detector such as a fluorescent screen FS. Thus, an ion image is obtained with the certain mass.
In the ion optics shown in FIG. 1, electrostatic lenses L.sub.11 and L.sub.12 are used to form the primary ion beam. The transfer optics TO consists of electrostatic lenses L.sub.21, L.sub.22, L.sub.23. A slit SL.sub.1 is disposed at the entrance to the mass analyzer MS. The ion optics further include an intermediate lens L.OMEGA., an energy slit SL.sub.2, and a mass selecting slit SL.sub.3.
In the instrument shown in FIG. 1, the secondary ions emitted from the sample surface have a large energy spread and, therefore, the mass analyzer MS consists of a double-focusing mass analyzer in which a spherical electric field EF and a uniform sector magnetic field MF are connected in tandem. The direct imaging type SIMS instrument as shown in FIG. 1 is disclosed in pages 17-19 in the third chapter (III. DIRECT IMAGING INSTRUMENTS) in an article "Microanalyzers Using Secondary Ion Emission" by George Slodzian in a book Applied Charged Particle Optics, 1980.
In order to satisfy the double-focusing condition for the ion optics, energy aberrations must be made zero for both the crossover and the ion image. To reduce the energy aberration regarding the ion image, (1) an image of the bombarded sample region is formed at the position of the first principal plane of the spherical electric field by the transfer optics TO, (2) the image which has been shifted onto the second principal plane by a spherical electric field is brought onto the first principal plane of the magnetic the magnetic field by the intermediate lens L.OMEGA., and (3) the image which has been shifted onto the second principal plane by the magnetic field is projected onto the screen FS by the projector lens Lp.
To reduce the energy aberration regarding the crossover down to zero, (4) the crossover of an image of the bombarded sample region is brought into the position of the entrance slit SL.sub.1 by the transfer optics TO, (5) the crossover formed at the position of the energy slit by the spherical electric field is moved by the intermediate lens L.OMEGA., and (6) the crossover is imaged at the position of the mass-selecting slit SL.sub.3 by the magnetic field.
For the aforementioned ion optics, it is inevitable that the mass analyzer is large and complex, because it comprises the tandem arrangement of the electric field, the lens L.OMEGA., and the magnetic field. Also, an adjusting operation for meeting the above-described conditions (1)-(6) for the image and the crossover needs skillfulness and a long time.
In the conventional optics described above, only a mass-filtered ion image is obtained. In such mass-filtered ion image, only selected ions having the specified mass contribute to the formation of the ion image. Therefore, it is impossible to derive any information from such ion image about the other ions not having the specified mass.